Crowds today gathered around Ground Zero in New York City, the damaged Pentagon in Virginia, and a now hallowed field in Pennsylvania to mourn the 3,025 people from 91 countries killed in the terrorist attacks one year ago.

At 8.46am (1.46pm BST) New York time, the city's governor, Michael Bloomberg, stood on the site of the wreckage to lead the nation in a moment of silence that marked the moment the first plane slammed into the World Trade Centre.

Troops at Bagram air force base in Afghanistan, relatives of the British victims at St Paul's cathedral in London, and mourners joining the US president in Washington DC also fell silent.

The former mayor, Rudy Giuliani, read out the names of the New York victims.

At the Pentagon, the US president, George Bush, and the defence secretary, Donald Rumsfeld, unfurled a massive flag at a memorial ceremony. The ceremony included a moment of silence, a playing of the national anthem, a recitation of the pledge of allegiance and an air force flyover.

"One year ago, men and women and children were killed here because they were Americans and because this place is a symbol to the world of our country's might and resolve," Mr Bush said.

"Today, we remember each life."

Standing outside the newly restored wall on the west side of the Pentagon, Mr Bush said: "The memories of a great tragedy linger here, and for all who knew loss here, life is not the same. The murder of innocents cannot be explained, only endured. And though they died in tragedy, they did not die in vain."

By daybreak the streets surrounding the site of the former World Trade Centre were bustling with people - many carrying American flags or wearing stars-and-stripes shirts, ties and bandanas.

Before sunrise, firefighters and police - who lost 423 colleagues on September 11 - marched across the city accompanied by bagpipes and drums, as bystanders shouted their thanks.

Nerves were strained on an already tense day by security alerts warning Americans that terrorist "chatter" had increased in the days leading up to today's memorials. Fighter planes patrolled the skies and live missiles stood at the ready in New York and Washington.

Stock markets on Wall Street, deeply affected by the attacks and shoulder-deep in ash and dust for days afterwards, remained closed for the morning.

At the Pentagon, troops in full battle dress with automatic rifles guarded the building. More than 13,000 people are expected to turn out for a memorial ceremony at the Phoenix Project construction site, where workers are still repairing the section of building that was damaged.

There, too, there will be a moment of silence, as well as the national anthem, a recitation of the pledge of allegiance and an air force flyover.

Near Ground Zero one man clutched a coffin holding an effigy of Osama bin Laden which he had towed on the back of his Harley Davidson all the way from South Carolina.

"I wanted to remind the world how evil this guy is," said Mitch Mitcham. "If he's not dead yet, we're going to get him soon."

Several people at the site had been there all night holding vigils to remember the dead. Ali Nichols, 19, who is from Texas but studies at Pace University a few blocks from Ground Zero, had been there with her four friends since midnight.

The group, who were all wearing I Love New York T-shirts, said they wanted to show their support for the city and the victims' families.

"This time last year there was smoke coming into the building where we were studying and we had to evacuate. It was a terrifying experience," said Ms Nichols. "It's been a remarkable year in New York. I hope being here today might help the city come to some sort of closure on what has happened."

Construction workers were carrying out a last clean-up operation at the bottom of the seven-storey crater in preparation for a memorial ceremony, which will see the families descend for the first time into the 16-acre site where their loved ones died.

Church bells will toll at 10:29 am local time, to mark the moment when the second tower collapsed. Relatives of many of the 2,801 victims in New York will carry roses onto the site. President Bush will address the country from Ellis Island at 9pm.